Do you want to test API or create API? If you want to create API probably you need to checkout different resources such as swagger. If you want to get started with testing and understand the tool, https://support.smartbear.com/readyapi/getting-started/
Please read this article "https://smartbear.com/resources/videos/create-your-first-rest-test-api-testing-with-ready"MyGiftCardSite , It's may help you for solving your issue.
